In the analysis of chemical vapors there is frequently a need to detect extremely small trace amounts of a specific vapor amongst a background of different chemical species. Examples are the detection of contraband aboard the cargo of vessels being inspected by the United States Coast Guard and the detection of leaking explosive or dangerous chemicals at depositories thereof. Automated or portable testing apparatus for this purpose has generally not been available. Further, existing detectors are only able to detect chemical species at concentrations well above their ambient vapor concentrations and thus lack sufficient sensitivity.
Chemical sensors have been developed that physically change upon exposure and contain absorbing polymers selected for their affinity to absorb a group of related chemical species. One type, surface acoustic wave delay line sensors, are the most developed and readily available, for example, one commercial supplier is Microsensor Systems, Inc., Fairfax, Va.
A method and apparatus for using A SAW device to detect a vapor is disclosed by H. Wohltjen in U.S. Pat. No. 4,312,228, issued Jan. 26, 1982. As described therein, the SAW device comprises a piezoelectric element having a surface coated with a polymer material selected to absorb and react with the chemical to be detected. Interaction of the chemical with the material coating of the sensing element alters one or more properties of a surface acoustic wave, and the electrodes on the piezoelectric element detect the altered wave, producing an electrical signal.
Another apparatus and method for detection and identification of chemical vapors is disclosed in U.S. Pat. No. 4,895,017. As described therein a plurality of surface acoustic wave (SAW) devices, each coated with a selected polymer material are exposed to the vapor to be analyzed. In this invention a predicted time constant (or rate) of diffusion into the polymer coating is used to identify the different chemical species. To quantitatively identify specific chemical species present in vapors an array of SAW sensors with different polymer coatings may be exposed and a pattern recognition technique utilized to identify specific species. This is described in a paper entitled "Correlation of Surface Acoustic Wave Device Coating Responses With Solubility Properties and Chemical Structure" by D. S. Ballentine, Jr., S. L. rose, J. W. Grate, and H. Wohltjen, published in Analytical Chemistry, Vol. 58, P. 3058, December 1986.
A further patent using multiple polymer coated dispersive delay lines is disclosed by J. Haworth in U.S. Pat. No. 5,012,668, issued May 7, 1991. The use of specific absorbant polymers to sensitize the surface of a piezoelectric crystal and induce a phase or amplitude variation in a traveling acoustic wave is common to all of the prior art and this approach severely limits the performance of these vapor detectors. Multiple polymer films dilute the vapor samples and thereby limit the amount of vapor that can be detected by each film. Also, practically any type of film applied to the surface of a piezoelectric crystal introduces noise which reduces sensitivity further. None of the prior art provides an efficient method of transporting the vapors being analyzed to the surface of the sensing crystal. Nor does any of the prior art utilize the desorption characteristics of vapor species to identify individual vapor species.
There are Four problems with the procedures used for identifying chemical substances in the above described tests. First, the prior methods rely upon determination of the rate of diffusion and or equilibrium concentration of a chemical vapor absorbed into a coating on the surface of a SAW device. The time to identify a chemical species by chemical absorption of that species into a polymer film is too long for applications such as high speed chromatography, which require identification to be in seconds or milliseconds.
Second, previous polymer coated sensors and sensor arrays require that they be coated uniformly to insure unbiased readings. This requires that input vapor elements be diluted and divided between the absorbing polymer coatings equally. In many cases only trace amounts of material are available and therefore cannot be equally exposed to the sensors without an undesirable reduction in the sensitivity of the vapor detection apparatus.
Third, previous sensors do not interact directly with a well defined gas jet stream and require a manifold or distribution system to pass vapors to be detected to the acoustic sensor containment vessel, typically a vented TO-8 style integrated circuit package. This allows vapor containing gas to contact a significant amount of equal temperature surface areas not associated with the detection process. These surfaces adsorb and trap condensible vapors and reduce the amount of vapor material which can reach and be absorbed by the sensor's polymer coating.
Fourth, previous sensors require polymer coatings selective to a given species. These coatings introduce dissipation, attenuation and loss into the propagation path of the surface wave device and this causes lower signal to noise in the measurement of the surface wave velocity, phase, or amplitude. The low specificity of the polymer coating and sample dilution together increased noise and decreased sensitivity can cause an unacceptable number of false alarms.